Fetal response decrement: true habituation?

Preliminary evidence suggests that fetal movement in response to vibrotactile stimulation "habituates" with repeated exposure. If demonstrated in fetuses, the habituation paradigm may be useful as a measure of fetal well-being or as a predictor of neonatal outcome. However, the ability of the fetal response decrement phenomenon to meet criteria for habituation which would distinguish it from simple receptor fatigue has not been evaluated. In this study, fetuses were exposed to two different frequencies of vibration applied to the maternal abdomen. Fetal movement was observed on an ultrasound monitor. An inverse relationship between the strength of the stimulus and rapidity of the response decrement was not observed. However, return of the response to presentation of a novel stimulus, response decrement to repetition of the second stimulus, and more rapid response decrement upon re-presentation of the original stimulus suggest that the fetal response decrement phenomenon is true habituation.